There is often only a single layer of 6 to 10 coccoliths, but coccospheres can have multiple layers of coccoliths.

The coccoliths have a distinctive perforate appearance since the distal-shield elements are hammer-shaped with gaps between them.

Slits can also occur between the proximal shield elements and there is usually a grill of delicate elements in the central area.

The entire coccolith is formed of a single cycle of 20 to 40 crystal units with radial calcite c-axes.

Monomorphic coccospheresSome coccolithophores form their coccospheres from 2 or more different types of coccoliths (Young et al. 1997). Monomorphic coccospheres, like those of E. huxleyi, contain only one type of coccolith.

Placolith coccolithsCoccolith shape is highly variable between coccolithophore species, but the most common type are placoliths. These are rivet-shaped coccoliths formed of 2 flange-like shields separated by a tube. This is an efficient design, since the individual coccoliths slot together to form a robust coccosphere which can grow by insertion of extra coccoliths.

Multiple layers of coccolithsOne of the unusual features of E. huxleyi is that it can produce multilayered coccospheres. Specimens can often have 50 or more coccoliths.

Crystal unitsThe structure of individual E. huxleyi coccoliths is best seen in broken specimens, when it can be seen that the different elements of the proximal (lower/inner) shield, distal (outer/upper) shield and tube and central area are interconnected and all formed from a single, rather complex-shaped, calcite crystal-unit. An intriguing complication of this basic structure is that the tube is formed of distinct inner and outer layers with the elements sloping in opposite directions. These lock the crystal units together so that the entire coccolith does not fall apart, like a chocolate orange, but forms a robust structure. Overall the coccolith shape and structure is a remarkable example of bioengineering in miniature.

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Introduction

Dip a bucket in the ocean almost anywhere in the world and you will recover a few hundred or maybe tens of thousands of cells of the coccolithophore Emiliania huxleyi. It is one of the most beautiful and widespread unicellular organisms, and in early summer it forms enormous blooms around the edges of the northwest european shelf.

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Comprehensive Description

Biology

prominent membrane-bounded organelles including chloroplasts (the site of photosynthesis)

the nucleus (where the DNA occurs)

mitochondria (the site of cellular biosynthesis)

a golgi body

an endoplasmic reticulum

The most distinctive aspect is that the coccoliths are formed inside the cell, in a 'coccolith vesicle'. After the coccolith has formed this vesicle migrates to the edge of the cell and fuses with the cell membrane so that the coccolith is extruded into the coccosphere.

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Ocean acidification

Emiliania huxleyi has a global population of about 7 x 1022 cells (Emiliani, 1993) scattered across the world ocean and adapted to a wide range of environments. So it should be one of the species least in danger from human activities. Ocean acidification, however, poses a potential threat to all marine organisms which produce calcareous skeletons.Burning of fossil fuels and deforestation is releasing immense volumes of carbon dioxide into the atmosphere, and around 25% is then dissolved into the sea. This is shifting the chemistry of sea water, making it slightly more acidic and making calcification more difficult.Organisms such as corals and pteropods (planktonic snails) which produce shells made of aragonite - a less stable calcium carbonate mineral - are likely to be affected by ocean acidification before organisms like coccolithophores which use calcite.If carbon dioxide emissions are not reduced, however, it is quite possible that by the end of the century even E. huxleyi will be in ecological trouble.

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Associations

Red Queen or Cheshire Cat?A series of studies (for example, Bratbak et al. 1993, Martinez et al. 2007) have shown that towards the end of Emiliania huxleyi blooms the cells are infested by large viruses, termed E. huxleyi viruses. The build-up of these viruses kills the coccolithophores and ends the blooms. It has been suggested that these viruses have helped drive the rapid evolution shown by E. huxleyi and closely related coccolithophores. This would be a classic case of Red Queen evolutionary dynamics (van Valen, 1973), named after the Red Queen in Alice in Wonderland who found, 'It takes all the running you can do, to keep in the same place'.However, an elegant study by Frada et al. (2009) suggests an alternative model, which they named 'Cheshire Cat dynamics' after the cat in Alice in Wonderland which escaped execution by gradually turning invisible. This study showed that the viruses only infect the diploid, coccolith-bearing, stage of the lifecycle. The alternate haploid stage of the life-cycle appears to be immune to the virus, possibly because it has a very different cell-surface and so is in effect invisible to the virus. The researchers speculated that at the end of blooms a proportion of the population escapes the viruses by switching to the alternate lifecycle phase.

Population biology

Why is E. huxleyi so abundant?Possibly E. huxleyi has evolved unique features which enables it to out-compete other species in a wide range of environments.Intriguingly, the species shows enormous variation in the ratio of coccolith calcite to cellular biomass as a result of the:

open structure of its coccoliths, which allows great variation in the mass of individual coccoliths

very variable number of coccoliths borne by individual cells

While most coccolithophore species consistently produce a single layer of coccoliths, E. huxleyi frequently over-produces coccoliths. This results in the formation of multi-layered coccospheres and/or the release of numerous coccoliths into the water column. Possibly this ability to calcify to varying degrees has given E. huxleyi a unique adaptive advantage.

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Distribution ecology

Vertical distributionLike all coccolithophore species, Emiliania huxleyi is confined to the upper part of the water column where light allows photosynthesis. The depth of this zone varies with oceanic conditions, but typically E. huxleyi only occurs in the top 50-100m of the water column.

As a result, there are clearly different assemblages of coccolithophores in, for instance, the surface waters of oligotrophic gyres, the deep photic zone and in eutrophic upwelled waters (e.g. Winter et al. 1994). E. huxleyi, however, occurs almost everywhere:

from near-freezing sub-Arctic waters to the equator

from extreme nutrient depleted waters to upwelling zones

from the surface to below the deep chlorophyll maximum

A one litre water sample taken from the photic zone of the ocean virtually anywhere in the world is likely to contain numerous specimens of E. huxleyi, with abundance varying from several hundred cells in low-productivity, mid-ocean waters to a few million cells per litre in blooms.

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Life History and Behavior

Behavior

Behaviour

BloomsIndividual Emiliania huxleyi cells can only be seen with the help of microscopes, but periodically they occur together in enormous blooms which can be clearly seen from satellites. Algal blooms occur when the conditions are ideal for the species forming them resulting in explosive population growth. In the case of E. huxleyi this can lead to populations of 1 to 10 million cells per litre, and in addition hundreds of millions of loose coccoliths floating in the water. This turns the water milky white, in contrast to green or red water produced by other phytoplankton species. A few other coccolithophores are known to form blooms but E. huxleyi is by far the most important bloom forming species.E. huxleyi blooms typically occur in early summer, in temperate waters along the edge of the continental shelf. Areas with regular blooms include:

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Reproduction

The most common mode of reproduction in E. huxleyi and other coccolithophores is asexual binary fission. One cell divides in two, by mitosis, producing two identical daughter cells. If nutrient elements and light are abundant then E. huxleyi cells will divide about once a day, allowing very rapid population growth (Paasche et al., 2002). Under less favourable conditions, however, a single cell can survive without reproducing for several weeks at least.In addition, E. huxleyi has a dimorphic lifecycle with alternation of a diploid phase (with two copies of each chromosome) and a haploid phase (with one copy of each chromosome). This alternation of phases, via meiosis and syngamy, allows genetic recombination and maintains diversity. The diploid phase is non-motile and forms the distinctive coccoliths. The haploid phase is motile, it has two flagellae which allow it to swim. In E. huxleyi, this phase does not produce coccoliths, but does produce cellulosic scales. An unusual feature of coccolithophores which is shared by E. huxleyi is that both the haploid and the diploid phase can reproduce asexually so that the one species can exist indefinitely in either of two forms.

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Evolution and Systematics

Evolution

Emiliania huxleyi coccoliths appear in the fossil record only 240-280,000 years ago, in Marine Isotope Stage 8 (Thierstein et al. 1977). Although the first occurrence can be hard to place since the coccoliths are small and initially only present at low abundances, it is has been reliably identified in over 25 well-dated sediment cores from around the world and consistently occurs within this interval (Raffi et al. 2006). E. huxleyi coccoliths occur at subsidiary abundances until the last glacial interval (MIS2-4) when they increase in abundance. They typically dominate assemblages from the Holocene (MIS1).The recent first occurrence makes E. huxleyi the youngest coccolithophore species for which we have a reliable fossil record, as well as the most abundant and ecologically widespread species.The fossil record, however, indicates that E. huxleyi is merely the latest of a succession of closely-related species which have occupied a similarly dominant position in coccolithophore assemblages. For example Gephyrocapsa caribbeanica dominates fossil assemblages globally during marine isotope stages 14 to 8, 480-260,000 years ago (Bollmann et al. 1998). One suggestion for this succession of short-lived dominant species is that co-evolution of coccolithophores and their associated viruses is driving rapid evolution in the dominant coccolithophores (Emiliani 1993, Smetacek 2001).

Emiliania huxleyi, often abbreviated "EHUX", is a species of coccolithophore with a global distribution from the tropics to subarctic waters. It is one of thousands of different photosynthetic plankton that freely drift in the euphotic zone of the ocean, forming the basis of virtually all marine food webs. It is studied for the extensive blooms it forms in nutrient-depleted waters after the reformation of the summer thermocline. Like other coccolithophores, E. huxleyi is a single-celled phytoplankton covered with uniquely ornamented calcite disks called coccoliths (also informally known as liths or scales). Individual coccoliths are abundant in marine sediments although complete coccospheres are more unusual. In the case of E. huxleyi, not only the shell, but also the soft part of the organism may be recorded in sediments. It produces a group of chemical compounds that are very resistant to decomposition. These chemical compounds, known as alkenones, can be found in marine sediments long after other soft parts of the organisms have decomposed. Alkenones are most commonly used by earth scientists as a means to estimate past sea surface temperatures.

Contents

Emiliania huxleyi was named after Thomas Huxley and Cesare Emiliani, who were the first to examine sea-bottom sediment and discover the coccoliths within it. It is the most numerically abundant and widespread coccolithophore species. Its coccoliths are transparent and commonly colourless, but are formed of calcite which refracts light very efficiently in the water column. This, and the high concentrations caused by continual shedding of their coccoliths makes E. huxleyi blooms easily visible from space. Satellite images show that blooms can cover large areas, with complementary shipboard measurements indicating that E. huxleyi is by far the dominant phytoplankton species under these conditions. This species has been an inspiration for James Lovelock's Gaia hypothesis which claims that living organisms collectively self-regulate biogeochemistry and climate at nonrandom metastable states.

E. huxleyi is by far the most abundant coccolithophore found in the Earth's oceans, and is considered ubiquitous, occurring everywhere except the polar regions. During massive blooms (which can cover over 100,000 square kilometers), EHUX cell concentrations can outnumber those of all other species in the region combined, accounting for 75% or more of the total number of photosynthetic plankton in the area. EHUX blooms regionally act as an important source of calcium carbonate and dimethyl sulfide, the massive production of which can have a significant impact not only on the properties of the surface mixed layer, but also on global climate. The blooms can be identified through satellite imagery because of the large amount of light back-scattered from the water column, which provides a method to assess their biogeochemical importance on both basin and global scales. These blooms are prevalent in the Norwegianfjords, causing satellites to pick up "white waters", which describes the reflectance of the blooms picked up by satellites. This is due to the mass of coccoliths reflecting the incoming sunlight back out of the water, allowing the extent of EHUX blooms to be distinguished in fine detail.

Extensive E. huxleyi blooms can have a visible impact on sea albedo. While multiple scattering can increase light path per unit depth, increasing absorption and solar heating of the water column, EHUX has inspired proposals for geomimesis,[1] because micron-sized air bubbles are specular reflectors, and so in contrast to E. huxleyi, tend to lower the temperature of the upper water column. As with self-shading within water-whitening coccolithophore plankton blooms, this may reduce photosynthetic productivity by altering the geometry of the euphotic zone. Both experiments and modeling are needed to quantify the potential biological impact of such effects, and the corollary potential of reflective blooms of other organisms to increase or reduce evaporation and methane evolution by altering fresh water temperatures.

As with all phytoplankton, primary production of EHUX through photosynthesis is a sink of carbon dioxide. However, the production of coccoliths through calcification is a source of CO2. This means that coccolithophores, including EHUX, have the potential to act as a net source of CO2 out of the ocean. Whether they are a net source or sink and how they will react to ocean acidification is not well understood.

Scattering stimulated by EHUX blooms not only causes more heat and light to be pushed back up into the atmosphere than usual, but also cause more of the remaining heat to be trapped closer to the ocean surface. This is problematic because it is the surface water that exchanges heat with the atmosphere, and EHUX blooms may tend to make the overall temperature of the water column dramatically cooler over longer time periods. However, the importance of this effect, whether positive or negative, is currently being researched and has not yet been established.

Dambara, A.; Y. Shiraiwa (1999). "Requirement of selenium for the growth and selection of adequate culture media in a marine coccolithophorid, Emiliania huxleyi". Bulletin of the Society of Sea Water Science, Japan53 (6): 476–484.